A TELEPHONE SYSTEM AND ADAPTER THEREFOR

Abstract: A telephone adapter (21), comprising an outlet (32) for coupling to a telephone outlet socket having a d.c. voltage across a telephone line connected to a pair of terminals thereof, an inlet (34) for coupling to a telephone extension unit (20), and a switch connected between the outlet (32) and the inlet (34) and operable between a first position in which the telephone extension unit (20) is directly connected to the telephone outlet socket (33) and a second position in which the extension coupling means is a.c. coupled to the line coupling means so as to block said d.c. voltage when the adapter (21) is connected to the telephone outlet socket. The invention also contemplates a system comprising several adapters each connecting respective telephone extension units to a common telephone line, whereby a respective control unit operating in accordance with suitable logic permits connection of one extension unit to the telephone line (11) or, alternatively, to another internal extension unit. Such a system operates as a PABX exchange without requiring a line interface where the telephone line (11) connects to the extension units.

Description: A Telephone System and Adapter Therefor FIELD OF THE INVENTION This invention relates to telephone systems of the kind permitting at least one external telephone line to be connected to a plurality of internal users, one of whom may make external calls whilst still allowing the remaining internal users to make internal calls. BACKGROUND OF THE INVENTION Small telephone exchanges, such as are commonly provided in offices and the like, permit a plurality of internal users to make outside calls via one or more outside lines and also to use the system as an internal intercom whereby they can call one another internally. In such systems, each internal extension has a unique identity code which must be dialled in order to call the respective extension. Normally, some special number, such as "9", must be dialled in order to obtain an outside line. A large number of such systems exists, each providing a variety of features, but all existing systems require that the internal exchange is coupled to the outside line and then to each of the internal extensions separately in a radial arrangement. This requirement renders the installation of such systems costly, since each internal extension must be separately wired to the internal exchange, an operation which is labor-intensive, time- consuming and inconvenient. Many small businesses and domestic installations have extension telephones which are commonly coupled to a single outside line. This provides the convenience of being able to answer the telephone in more than one location, but does not provide an intercom facility whereby a user in one room can call another user in a different room via the telephone network. So far, the only way to provide such a facility has been to install an internal exchange of the type described above: a fact which has clearly militated against the widespread use of such systems, particularly for domestic situations. Systems have been proposed for at least partially addressing this requirement. For example, U.S. Patent No. 4,821,319 (Middleton et al.) discloses a single line telephone communication system for the provision of four intercom services and/or PABX-type features to a plurality of subsets. Each subset is provided with a station adaptor adapter for coupling it to a single pair of in-house wires and additionally there is required a line interface adapter connected between the in-house wires and an external line. The arrangement proposed by Middleton et al. obviates the need for much of the complex wiring associated with other systems, since each telephone subset is connected in parallel to the in-house wires and does not need to be separately wired to the line interface. There is, thus, only one internal connection to the line interface. This notwithstanding, the line interface itself must be connected to the external line, and this cannot be done without installing the system: something which is probably beyond the capabilities of the average domestic user. U.S. Patent No. 4,893,334 (Parnello) discloses an intercom system for use with a plurality of telephones on a single extension, including a single master station and a plurality of slave stations. The master station is connected so as to interrupt the telephone line pair to and from a central exchange. Each slave station is preferably associated with a handset that is on the extension. When the extension is not being used for intercom service or for an outside call, any telephone handset on the extension that has a keypad can initiate a signal that is received and decoded in the master unit. When the master station receives such a signal, it disconnects the line to the telephone exchange from the extension and connects the telephone exchange to a circuit that detects incoming calls so that they can be signalled to users. The master station also sends a signal to each slave station that enables the intercom function. In such an arrangement, the master station serves a similar function to the line interface shown by the Middleton et al. in that it must be connected between the external telephone wires and the internal system. SUMMARY OF THE INVENTION It is an object of the present invention to provide a telephone system, particularly one suitable for small business and residential use, which requires no interface with the incoming telephone line and therefore obviates the need for specialized installation. According to the invention, there is provided a telephone system, comprising: at least two telephone outlet sockets coupled to a telephone line across which there is applied a d.c. voltage, a respective telephone extension unit for coupling to the telephone line, add a respective switching means connected between each one of the telephone outlet sockets and the corresponding telephone extension unit coupled thereto and switchable between a first position in which the telephone extension unit is directly connected to the telephone outlet socket and a second position wherein the telephone extension unit is a.c. coupled to the telephone outlet socket so as to block said d.c. voltage from the telephone extension unit; the system further including in respect of each telephone extension unit: call signalling means for sending a call signal to another telephone extension unit, a call signal detector for detecting said call signal sent by a remote telephone extension unit and producing a call detection signal, a call indication means responsive to the call detection signal for generating a call indication signal, a status indication means responsive to a status of the system for producing a respective status indication signal, an actuation means for producing an actuation signal, and a control unit coupled to the status indication means, the actuation means, the call signal detector, the call indication means, the call signalling means and the switching means and being responsive to the status indication, the actuation and the call detection signals, for operating the call indication means, the call signalling means and the switching means. The invention also incorporates within its scope an adapter which may be connected between each extension telephone and its respective telephone outlet socket in a conventional multi-extension system sharing a common telephone line, so as to convert all of the telephone extension units thus connected to a telephone system having all of the features of a conventional PABX system. Such an adapter incorporates therein a control circuit for providing all of the facilities normally associated with such systems and therefore necessarily duplicates some of the components which are in any case provided in more sophisticated telephones. Alternatively, the telephone itself may be modified so as to include therein all of the special features of the adapter, in which case the adapter can be dispensed with. BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how the same may be carried out in practice, some preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing functionally a system according to the invention; Fig. 2 is a circuit diagram showing schematically an adaptor for use with the system shown in Fig. 1; Fig. 3 is a block diagram illustrating how a controller shown in Fig. 2 may be incorporated into the system shown in Fig. 1; and Figs. 4 to 10 are flow diagrams showing the principal operating steps associated the adaptor shown in Fig. 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows a system, designated generally as 10, comprising three pairs of telephone lines 11, 12 and 13 connected to a central office exchange 14. The telephone lines 11, 12 and 13 are commonly connected to a telephone 15 via an adaptor 16 and, in similar manner, an answering machine 17 is connected to the telephone lines 11, 12 and 13 via an adaptor 18. The telephone line 11 corresponds to the normal tip and ring lines which are commonly connected to multiple-extension telephone sets in a domestic installation, wherein only one telephone extension may initiate an external telephone call, no intercom facility being provided. Such a connection is represented by a telephone 19 which is directly connected to the telephone line 11. In contrast to this, there is shown a telephone 20 which is connected to the telephone line 11 via an adaptor 21. Likewise, a telephone-operated door lock 22 is connected to the line 11 via an adaptor 23; a modem 24 is connected to the telephone line 11 via an adaptor 25; a fax machine 26 is connected to the telephone line 11 via an adaptor 27 and remotely-operated appliances 28 and 29 are connected to the telephone line 11 via an adaptor 30. A control unit 31 is commonly coupled to the telephone line 11 for controlling the sequence in which all automatic extension units are controlled as will be explained in greater detail below with particular reference to Fig. 3 of the drawings. All of the adapters shown in Fig. 1 are provided with an outlet (constituting a line coupling means) shown schematically as 32 for coupling to a telephone outlet socket designated schematically as 33 and is also provided with an inlet (constituting an extension coupling means) designated schematically as 34 for coupling to the corresponding telephone. Thus, each adaptor is connected to the telephone line 11 by unplugging the correspond¬ ing telephone or other appliance therefrom, connecting the adaptor in its place via the line coupling means 32 and then connecting the telephone or other appliance. All of the adapters are substantially identical but for minor modifications which are dictated by the type of appliance connected thereto. Thus, the adaptor 21 through which the telephone 20 is connected to the line 11, is provided with an internal ringer whilst the adaptor 16, by means of which the telephone 15 is connected to one of the three lines 11, 12 and 13, has no internal ringer. Similarly, the adaptor 23, by means of which the door lock 22 is controlled, also has no internal ringer but is provided with a controlled switching device (not shown). Referring now to Fig. 2 of the drawings, a particular configuration of the adaptor 27 shown in Fig. 1 will now be described. Tip and ring connections of the incoming telephone line 11 are connected, via the line coupling means 32 (see Fig. 1), to respective diodes 40a and 40b, across which are coupled respective opto-couplers 41a and 41b such as manufac¬ tured by the Motorola Company under the catalog name 4N28. The emitters of the opto-couplers 41a and 41b are connected, via respective rectifier diodes 42a and 42b to a common bus 43. A DTMF Transceiver 44 is connected to the bus 43 and is also connected across the telephone line 11 via respective capacitors 45 a and 45b. Similarly, a ring detector 46 is connected to the bus 43 and is also connected across the telephone line 11. The telephone extension unit 20 is coupled across the telephone line 11 via a pair of normally-closed 2-pole, 2-throw switch contacts 47a and 47b, respectively operated by a relay A. Corresponding normally-open contacts 48a and 48b are connected to a ring driver 49 which is also connected to the common bus 43. The normally-open contact 48a is also connected to a Local Off Hook Detector 50 which is coupled to a power supply 51 via a primary winding of a coupling transformer 52. A Ring Notch Filter and Ring Tone Generator 53 is connected across the secondary winding of the coupling transformer 52 and is also connected, via a pair of normally-open contacts 54a and 54b of a relay B across the telephone line 11, an a.c. coupling capacitor 55 being connected between the normally-open relay contact 54a and the telephone line 11 so as to de-couple the d.c. voltage from the Ring Notch Filter and the Ring Tone Generator 53 and hence form the telephone extension unit 20. The output of the Local Off Hook Detector 50 is coupled via a rectifier diode 56 to the common bus 43. Also coupled across the telephone line 11 is a bridge rectifier 57 having respective positive and negative output rails 58a and 58b. Connected across the positive and negative output rails 58a and 58b are two voltage dividers comprising, respectively, resistors 59a, 59b and 60a, 60b. An inverter 61 is connected at its input to the common connection between the resistors 59a and 59b, whilst an inverter 62 has an input connected to the common junction of the resistors 60a and 60b. Respective outputs of the inverters 61 and 62 are coupled to a micro-controller 63. The positive output rail 58a of the bridge rectifier 57 is connected to an Audio Signalling Device 64 which is responsively coupled to the micro-controller 63. A manually operated push-button switch 65 is connected to the micro-controller 63, as is an LED 66. A switch contact 67 is responsively coupled to the micro-controller 63 for operating the remotely-controlled appliance 28 (shown in Fig. 1). The system 10 is energized by a source of d.c. electricity typically provided via a rectified domestic a.c. supply. There is provided a back-up power supply shown schematically as 68, typically constituted by an internal battery, for powering the internal circuit in the event that the domestic a.c. supply fails. The operation of the circuit will now be explained. The diodes 40a and 40b, in conjunction with the opto-couplers 41a and 41b and the rectifier diodes 42a and 42b, constitute an Extension On/Off Hook Detector and Pulse Detector which is coupled across the telephone line 11 for detecting an off-hook condition in any one of the telephone extension units coupled to the line 11 and producing a general off hook signal. The diodes 40a and 40b are connected in parallel to the respective opto-couplers 41 and 41b so as to be plurality independent and the configuration is symmetrical with regard to the tip and ring connections so that the circuit functions correctly regardless of the order in which the tip and ring connections are connected to the line coupling means. The capacitors 45a and 45b serve for a.c. coupling the DTMF Transceiver 44 to the telephone line 11, whilst the capacitor 55 serves for a.c. coupling of a speech signal on the telephone line 11 to the Ring Notch Filter and Ring Tone Generator 53. Signalling an inter-communication between the telephone extension units, of which only the extension unit 20 is shown in Fig. 2, is achieved by a.c. coupling of audio signals, both speech via the capacitor 55 and DTMF via the capacitors 45a and 45b, to the telephone line 11. The a.c. coupling for inter-communication with the system 10 is accomplished by depressing the manual push-button switch 65 prior to the lifting of the handset. Thus, depressing the push-button switch 65 results in a.c. connection between the telephone extension unit 20 and the telephone line 11 via the transformer 52 and the capacitor 55. This is achieved by changing the state of the relay A whereby the telephone extension unit 20 is de¬ coupled from the telephone line 11 and connected instead to the normally- open relay contacts 48a and 48b. In this condition, the power supply 51 supplies d.c. speech voltage to the telephone extension unit 20 through the primary winding of the transformer 52. The push-button switch 65 having been depressed, a so-called "smart code" is transmitted and the handset of the extension unit 20 may now be lifted and an extension number of another internal extension unit may be dialled either in pulse or DTMF within a predetermined time period (e.g. five seconds) whereby the "smart code", followed by the dialled extension number is received by all other adapters coupled to the system via their respective DTMF Transceivers 44 and the a.c. coupling capacitors 45a and 45b. Only the dialled extension adaptor will a.c. couple its telephone extension unit, responsive to the dialled extension number, by changing the state of its relay B, whereupon its d.c. speech voltage will be supplied by the corresponding supply 51 within its own adaptor 27 instead of via the telephone line 11. At the same time, all the other extension units in the system whose respective adapters do not recognize the dialled extension number will, under control of their respective micro-controllers 63, change the state of their respective relays A so as to disconnect their extension units 20 from the telephone line 11 so as to prevent their eavesdropping on the internal conversation between the internal caller and the internally-dialled extension number. In a similar manner, other internal extension units within the system can be allowed to join an existing conversation, so as to allow conference calls, by any one of the existing participants pressing his push¬ button switch 65 and dialling another unit's extension number. If, subsequent to effecting an internal call, an external call is received, then a pilot tone is generated by the Ring Tone Generator 53 and will be heard by all internal participants, so as to indicate the presence on the line of an external call. As will be explained later with particular reference to Fig. 5 of the drawings, upon pressing the push-button switch 65, the status of the relays A and B is now changed whereby the relay B, having been previously connected, is now disconnected, whilst relay A, having been previously connected to the normally-open contacts 48a and 48b, is now connected to the normally- closed contacts 47a and 47b. By this means, the d.c. speech voltage is disconnected from all the participating internal extension units, which are now connected directly to the external line 11. By virtue of having disconnected the Ring Tone Generator 53, the ring tone is interrupted and by virtue of connecting the extension units directly to the telephone line 11, the external call will now be received. A light-emitting diode (LED) 66 is responsively coupled to an output of the micro-controller 63 so as to give a visual indication when, during an existing call, the current telephone extension unit 20 is on-hook whether in EXTERNAL or INTERCOM mode. In such condition, relay A in the current telephone extension unit 20 is switched such that the telephone extension unit 20 is d.c. de-coupled from the telephone line 11, whilst relay B is also disconnected so that the current telephone extension unit 20 is also a.c. de-coupled from the telephone line 11. This situation, (constituting a third position of the switching means) is thus indicated by the LED 20 when illuminated. D.c. power is supplied to the system via the power supply 68 which is a typically derived from a rectified a.c. domestic supply backed up by an internal rechargeable battery. In the event of a failure of the domestic a.c. supply, all of the adapters are connected through their respective extension units 20 to the telephone line 11 and, although the system 10 no longer functions as an internal exchange system, the extension units 20 will nevertheless continue to operate as regular extension units. The diodes 40a and 40b in conjunction with the opto-couplers 41a and 41b and the rectifier diodes 42a and 42b constitute an Extension Off Hook and Pulse Dial Detector 70 which serves two functions. First, it detects whether the extension unit 20 is off-hook. Secondly, when the telephone extension unit 20 employs pulse dialling, the dialled pulses are detected by the Extension Off Hook and Pulse Dial Detector 70 and fed to the micro-controller 63 which instructs the DTMF Transceiver 44 to transmit the dialled pulses in DTMF. The Local Off Hook Detector 50 detects a local off-hook condition when the extension unit 20 is connected in INTERCOM MODE. The bridge rectifier 57 in conjunction with the voltage dividers 59a, 59b and 60a, 60b and the inverters 61 and 62 constitute a General Off Hook and Zero Volt Pulse Detector 72, which generates a General Off Hook signal whenever a handset of any of the extension units within the system 10 is lifted, or a zero volt pulse was sent down the telephone line 11 by another extension unit. The transformer 52 in conjunction with the Ring Notch Filter and Ring Tone Generator 53, the Relay B and the capacitor 55 together constitute an a.c. coupling circuit 73 for connecting the extension unit 20 to the telephone line 11 when a connection to another internal extension unit is required during INTERCOM MODE. The rectifier diodes 42a, 42b and 56 constitute OR-gates for the Local Off/Hook Detector 50, the Extension Off Hook and Pulse Dial Detector 70, respectively, so that in the event of any of these detectors generating appropriate detection signals, a Local Off Hook signal is fed via the common bus 43 to the micro-controller 63. The voltage across the bridge rectifier 57 between the positive and negative output rails 58a and 58b thereof is approximately 50V d.c, this being the d.c. voltage across the telephone line 11. On the other hand, the voltage which must be supplied to the inverters 61 and 62, as well as to the micro-controller 63, should be no greater than approximately 5V d.c. If the handset of an extension unit is lifted, then the voltage across the telephone line 11, throughout the whole system 10, falls from the original value of 50V d.c. to approximately 10V d.c. Thus, this drop in voltage may be employed in order to determine a General Off Hook condition. In a practical embodiment reduced to practice, the values of the resistors 59a and 59b are respectively 50 MΩ and 5 MΩ, such that the voltage at their common junction is approximately 5 V when all the handsets are closed and the voltage across the telephone line 11 is therefore 50V d.c. In this situation, the voltage at the output of the inverter 61 is 0 (logic LOW). If now one of the handsets in the system is lifted, then the voltage appearing across the telephone line 11 falls from approximately 50V d.c. to approximately 10V d.c, whereupon the voltage appearing at the common junction of the resistors 59a and 59b falls to somewhat less than IV d.c and the output of the inverter 61 now goes to logic HIGH. Thus, the voltage divider 59a and 59b in conjunction with the inverter 61 constitute a General Off Hook Detector for producing a General Off Hook signal whenever any of the handsets of the extension units 20 in the system 10 is lifted. The value of the resistor 60a is 50 MΩ and the value of the resistor 60b is 30 MΩ. Thus, when all of the handsets are closed and the voltage across the telephone line 11 is 50V d.c, the voltage appearing across the common junction of the resistors 60a and 60b is approximately 30V d.c. However, such a voltage cannot exist in practice, because the inverter 62 connected to the common junction of the resistors 60a and 60b is a CMOS circuit, having internal protection diodes which clip the voltage to a maximum of 5V d.c. The high values of the resistors 60a and 60b ensure that only minimal current flows to the inverter 62 (so as not to damage the internal protection diodes) and under normal conditions, the voltage appearing across the common junction of the resistors 60a and 60b is therefore clipped to 5V d.c. If the voltage across the telephone line 11 now drops to 10V d.c, then the voltage at the input of the inverter 62 falls to 6V d.c. (from the theoretical value of 30V d.c.) and, consequently, the output of the inverter 62 remains LOW. Even if more than one handset in the system 10 is lifted simultaneously, resulting in the line voltage falling from 10V d.c. to a somewhat smaller value, the voltage appearing at the common junction of the resistors 60a and 60b is still sufficiently high to ensure that the output of the inverter 62 remains LOW. However, when any handset is lifted and, at the same time, the telephone line 11 is temporarily shorted, then the voltage appearing across the common junction of the resistors 60a and 60b falls to zero and, in this case, the voltage appearing at the output of the inverter 62 goes to HIGH. The telephone line 11 may indeed be temporarily shorted when the push¬ button switch 65 is depressed, under control of the micro-controller 63, or upon lifting or replacing the handset of the extension unit 20 in INTERCOM MODE. Thus, the voltage divider 60a, 60b in conjunction with the inverter 62 constitutes a 0-10 and 10-50V pulse detector which produces a detection signal whenever the push-button switch 65 is depressed and it is required to awaken the DTMF Transceivers in all of the remaining adapters. The switch 67 operates under control of the micro-controller 63 to control the remotely-controlled appliance 28 in order that the appliance can be enabled or disabled remotely by telephone. The audio signalling device 64 is typically constituted by a beeper, buzzer or loudspeaker to give an audio signal or a vocal announcement when addressed under control of the micro-controller 63. The DTMF Transceiver 44, the Ring Detector 46, the Ring Driver 49, the Local Off Hook Detector 50, the Ring Notch Filter and Ring Tone Generator 53 and the Audio Signalling Device 64 are all standard components well-known to those skilled in the art and therefore not requiring any further description. The operation and the programming of the micro-controller 63 will be described in detail below with particular reference to the flow diagrams shown in Figs. 4 to 10 of the drawings. It should be noted that in the adaptor 27 shown schematically in Fig. 2, the DTMF Transceiver 44 remains dormant (i.e. inoperative) until awakened by the zero volt pulse which is produced on depressing the push¬ button switch 65 or upon lifting or replacing the handset in INTERCOM MODE. It then operates under control of the micro-controller 63 to perform whatever function is required, whereafter it is again (under control of the micro-controller 63) disabled until subsequently reawakened by another pulse. This is necessary particularly when the adaptor 21 is operated by the internal battery 68 since, whilst most of the components in the adaptor 21 require very little current when not operational, the DTMF Transceiver 44 has a current drain of 10 Ma which would drain the internal battery 68 within approximately only two weeks. By rendering the DTMF Transceiver 45 operational only when required, the life of the internal battery 68 may be increased to approximately two years since the current drain of the remaining components is of the order of only 20-30μA. Additionally, by awakening the DTMF Transceiver 44 only when it is required to transmit or receive DTMF signals, it can be ensured that a DTMF signal sent over the telephone line 11 will have no effect unless the DTMF Transceiver 44 of the respective adaptor 21 has first been awakened by depressing the push-button switch 65. This is important in the situation where an internal user answers an external call and transfers the call to another internal extension by depressing the push-button switch 65 (so as to awaken the DTMF Transceiver 44) and then dialling the appropriate internal extension number. The corresponding DTMF code thus generated will not only be transmitted to the internal extension but will, in fact, also be transmitted over the telephone line 11. It would obviously be undesirable to cause the transmitted DTMF signal to alert an internal extension at the far end having the same extension address code as the local extension to whom the call is to be transferred. However, in practice, this cannot happen because although the DTMF signal is indeed transmitted over the telephone line 11, it is not received by the DTMF Transceiver at the far end since this remains dormant, no awakening signal having been transmitted thereto. Fig. 3 illustrates in more detail the controller 31 shown in Fig. 1 for allowing connection and control of all automatic answering extension units connected to the telephone line 11. The controller 31 can be coupled to a plurality of telephone lines, if desired. The controller 31 comprises a DTMF/PULSE line interface 75 connected to the telephone line 11. An Off Hook Detector 76 and a Ring Detector 77 are likewise coupled to the telephone line 11 as well as being connected to a micro-controller 78 to which is also coupled a DTMF receiver 80. A Fax/Modem Signal Detector 81 is connected to the telephone line 11 as well as to the micro-controller 78. The circuit is powered by a power supply 82 which can either be derived from a rectified domestic a.c. supply or, alternatively, can be derived from an internal battery back-up. The controller 31 operates as follows. The Ring Detector 77 detects an external ring on the telephone line 11 and awakens the micro- controller 78. The micro-controller 78 allows sufficient time, for example, four rings, for a local subscriber to lift the handset of the extension unit coupled thereto whereafter, if the local handset is not lifted, the controller 31 connects a 600Ω load across the telephone line 11 so as simulate an off- hook condition. A false ring signal, of different frequency, is generated by the DTMF/PULSE Line Interface 75 and transmitted along the telephone line 11. By such means, an external caller receives the impression that the extension is on-hook. However, if the remote caller is a machine, such as a facsimile or modem, it will ignore the false ring signal, will detect the off-hook state of the local extension unit and will start sending its handshake protocol signals. The Fax/Modem Signal Detector 81 in the controller 31 identifies the type of service required by the remote caller according to its handshake protocol signals in order to determine whether the remote caller should be connected to a local facsimile machine, computer modem, remotely-operated appliance, and so on. If none of the above signals is detected within a predetermined time, e.g. three seconds, the controller 31 assumes that the local answering machine service is required and sends an appropriate control signal to the micro-controller 78 which, in turn, produces an appropriate "smart code" for addressing the corresponding telephone extension unit. Figs. 4 to 10 describe in detail the logic in accordance with which the micro-controller 63 (Fig. 1) operates. Fig. 4 shows a MAIN routine (100) which is executed continuously in order to determine the status of the respective adapter and which branches to subroutines as and when required. Specifically, at step 101, an initialization procedure is invoked whereby the status of the relays A and B is checked and, if necessary, they are returned to their respective reset states as shown in Fig. 2. All internal flags are likewise reset. At step 102, the status of the General Off Hook Detector 72 and the Local Off Hook Detector 50 is determined. A negative response to step 102 implies either that an external handset has been lifted, in which case the General Off Hook Detector 72 produces a signal whilst the Local Off Hook Detector 50 does not or, alternatively, that no local handsets have been lifted, in which case no signal is produced by either the General Off Hook Detector 72 or the Local Off Hook Detector 50. In either case, at step 104 a check is determined as to whether the General Off Hook Detector 72 has produced a signal. If so, this implies that another handset within the system has been lifted, in which case control branches at step 105 to the EXTERN.AL SLAVE routine which is described in greater detail below with reference to Fig. 6 of the drawings. If at step 104 no signal is detected by the General Off Hook Detector 72, then at step 106 a check is performed to determine whether the push-button switch 65 in the local handset has been depressed. If so, then at step 107 a single beep is initiated and control branches to the INTERCOM MA.STER routine, which is described in greater detail below with particular reference to Fig. 7 of the drawings. If the push-button switch 65 has not been depressed, then at step 108 a check is determined as to whether a pulse has been detected and, if so, control branches at step 109 to the INTERCOM SLAVE routine which is described in greater detail below with particular reference to Fig. 8 of the drawings. If no pulse is detected, then at step 110 a check is determined as to whether a ring signal has been detected by the Ring Detector 46. If so, then at step 111 the "RING" flag is set for four seconds and control branches to the beginning of the loop at step 102. If, however, at step 102 signals are detected from both the General Off Hook Detector 72 and the Local Off Hook Detector 50, this means that the current handset has been lifted, which can be due to one of two reasons. Either the local handset wishes to initiate a call or, alternative¬ ly, an external call was received by the local handset and the call has been answered. Therefore, at step 112 the status of the "RING" flag is deter¬ mined which, as has been explained, is set as soon as the Ring Detector 46 detects a ring signal owing to an incoming call. If the "RING" flag is OFF, then control branches at step 113 to the EXTERNAL MASTER routine at step 200. Otherwise, control branches at step 114 to the EXTERNAL MASTER routine at step 202. Referring now to Fig. 5 of the drawings, the principal steps of the EXTERNAL MASTER routine 200 will be described. At step 201 the INTERCOM FLAG is reset, if necessary, and the push-button switch 65 is disabled for five seconds. It will be recalled from the description of Fig. 4 above, that step 201 is executed only if the "RING" flag is OFF, implying that the local handset wishes to initiate a call. As will be explained below, the function of the push-button switch 65 is to initiate internal calls and, in this mode, it must be depressed before the handset is lifted and not afterwards. Therefore, in order to avoid any possible confusion whereby a user may forget and depress the push-button switch 65 after lifting the handset, the push-button switch 65 is disabled for a short time period, so that inadvertent depression of the push-button switch 65 is of no conse¬ quence. However, after a five second time period, the push-button switch 65 is enabled once again so as to permit the user to transfer a call or to invoke conference calls during a conversation. At step 202 the status of the General Off Hook Detector 72 is determined and, if no signal is detected thereby, then at step 203 control branches to the MAIN routine at step 100, as shown in Fig. 4. If a signal is detected by the General Off Hook Detector 72, then at step 204 the status of the Local Off Hook Detector 50 is determined. If no signal is received thereby, then at step 205 the status of the General Off Hook Detector 72 is again determined in order to check for the possibility that the local handset was lifted and then immediately returned. Under these circumstances, no General Off Hook signal is detected and so, at step 205 A, control branches once again to the MAIN routine at step 100. If a General Off Hook signal is still detected at step 205, then control branches at step 205B to the EXTERNAL SLAVE routine described below with reference to Fig. 6 of the drawings. However, if a Local Off Hook signal is detected, this means that the local handset has not been replaced and so, at step 205C, a check is performed to determine whether a "smart code" has been received by the current adapter from a remote internal adapter. Such a "smart code" may be sent when a local subscriber lifts his handset for the purpose of making an outside call and finds that the line is engaged owing to the present external call. In this case, the other adapter is an EXTERNAL SLAVE and, as will be explained below with particular reference to Fig. 6 of the drawings, he can send an "alert" signal in order to warn the EXTERNAL MASTER that he wishes to use the outside line. In such case, the EXTERNAL MASTER has two options: he can either immediately depress the push-button switch 65 in order to initiate a "smart code" whereby the EXTERNAL SLAVE may immediately be connected; or, alternatively, a ten second grace period will elapse, whereafter the EXTERNAL SLAVE will be connected anyway. This logic is reflected at step 205E, where a ten second timer is set, during which period beeps are heard in the EXTERNAL MASTER'S adapter and at step 207 a check is performed in order to determine whether the EXTERNAL MASTER has depressed his push-button switch 65 in order to allow connection of the EXTERNAL SLAVE. If not, then the loop repeats from step 202. If, however, at step 207 the push-button switch 65 has been depressed or, alternatively, if at step 206 the ten second time period has terminated, then the beeps are terminated and the push-button switch is enabled so as to produce the effect of the push-button switch 65 having been depressed. Then, the output of step 207 is affirmative and at step 208 the beeps are terminated, DTMF is awakened and a "smart code" constituted by a pulse followed by e.g. "#" is generated so as to allow the EXTERNAL SLAVE to join the conversation. The ten second time delay thus provides discrete connection by an EXTERNAL SLAVE in the event that the EXTERNAL M^TER does not grant him prior entry by appropriate transmission of the "smart code". In effect, there are now two EXTERNAL MASTERS in the system corresponding to the current handset and the handset which has just joined the conversation. In the logic described so far, the second EXTERNAL MASTER joined the conversation at his initiative by sending an interrupt signal. However, according to an alternative scenario, the EXTERNAL MASTER, corresponding to the current handset who is engaged in an external call, may decide on his own initiative to involve another local handset. This is accomplished by depressing the push-button switch 65 in order to send a "smart code", after which the local extension number of the desired extension unit is dialled. At step 209, having already established that the push-button switch is depressed, having awakened DTMF and set the five second timer (at step 208), a check is now performed in order to determine whether the Local Off Hook signal is detected. This test is necessary, notwithstanding the fact that a Local Off Hook signal has already been detected at step 204 because if the current handset operates using pulse dialling, then the Local Off Hook signal will appear to oscillate between ON and OFF every 0.05 s. Thus, if at step 209 no Local Off Hook signal is detected, then at step 210 a check is performed in order to determine whether the local handset used pulse dialling and, if so, at step 211 the dialled code is converted from pulse to DTMF, the DTMF signal is sent down the line, DTMF is killed and control branches to step 202. Otherwise, control branches to step 212, which checks whether a General Off Hook signal is detected, as well as the Local Off Hook signal which has already been established at step 209. If not, then the current handset (which is the EXTERNAL MASTER) is the only handset in the system which is off-hook and control branches at step 213 to the MAIN loop 100 already described with reference to Fig. 4 of the drawings. However, if a General Off Hook signal is detected then, at step 214, a further check is determined as to whether the Local Off Hook signal has been detected. This further check is necessary because the current handset, having dialled another internal extension, may have done so in order to transfer the external call thereto and may therefore have now replaced his handset. In this case, of course, no Local Off Hook signal will be detected and the current handset changes status from EXTERNAL MASTER to EXTERNAL SLAVE and control branches at step 215 to the EXTERNAL SLAVE routine 300 which will be described in greater detail below with reference to Fig. 6 of the drawings. However, if at step 214 a Local Off Hook signal is detected, then at step 216 the status of the push-button switch 65 is determined. This will be depressed if the current handset is still in the process of transferring a call to another internal extension via DTMF and, in this case, control branches to step 208. Otherwise, at step 217, the elapsed time period is determined and if the five second time period has not yet elapsed control branches to step 209 and the loop repeats. Otherwise, when five seconds has elapsed, control branches to step 202. Referring now to Fig. 6 of the drawings, the EXTERNAL SLAVE routine 300 will be described. As has already been explained in some detail with reference to Fig. 5 of the drawings, an EXTERNAL SLAVE is any internal extension which is on-hook whilst a conversation is in progress between an outside caller and at least one EXTERNAL MASTER. The EXTERNAL SLAVE routine 300 allows for the possibility that the EXTERNAL SLAVE may interrupt the EXTERNAL MASTER as described previously or, alternatively, may be dialled by the EXTERNAL MASTER either for the purpose of transferring the external call to the EXTERNAL SLAVE or for allowing him to participate in the conversation. On entry to the EXTERNAL SLAVE routine 300, relay A in the EXTERNAL SLAVE adapter is reset such that the telephone extension unit 20 is connected to the telephone line 11. Therefore, relay A is now set at step 301 in order to disconnect the extension unit from the line whereupon, at step 302, the status of the General Off Hook Detector is determined. If no General Off Hook signal is detected, then the EXTERNAL MASTER which went off hook has subsequently gone on-hook and so at step 303 relay A is reset, the push-button switch 65 in the local adapter is enabled and control branches to the MAIN routine 100 described above with reference to Fig. 4 of the drawings. Otherwise, if a General Off Hook signal is detected, then the EXTERNAL MASTER is still connected and at step 302A the status of the push-button switch 65 in the local adapter is determined. If this is depressed, then at step 302B a single beep is emitted, followed by a "smart code" comprising a zero volt pulse followed by e.g. DTMF "A". A series of DTMF beeps is also produced so as to provide a status indication to the EXTERNAL SLAVE. At the same time, the smart code is transmitted to the EXTERNAL MASTER and causes a series of DTMF beeps to be produced therein so as to provide corresponding status indication to the EXTERNAL MASTER. As has been explained previously, a ten second timer is now initiated and a BREAK FLAG is set. Step 302C checks whether the ten second time limit has elapsed, in which case at step 302D, relay A is reset, thereby connecting the local handset to the line whereupon the local telephone extension unit becomes an EXTERNAL MASTER and control branches to step 202 (Fig. 5). If the ten second time limit has not yet elapsed, then at step 304 a check is determined as to whether a pulse has been received having been sent down the telephone line 11 from the EXTERNAL MASTER in order to connect the local extension unit to the conversation even before the ten second time limit has elapsed. If no such pulse is detected, then control branches to step 302 and the loop repeats. Otherwise, at step 305, DTMF is awakened for one second in order to allow sufficient time for an interrupt signal constituted by e.g. DTMF "#" sent by the EXTERNAL MASTER to be detected at step 306. Again, if no such signal is detected, control branches to step 302. Otherwise, at step 307 the status of the BREAK FLAG is determined and if it is set then, at step 307B, the DTMF beeps which were initiated at step 302B to give an indication to the EXTERNAL SLAVE are stopped, relay A is reset and control branches to the EXTERNAL MASTER routine at step 202 (see Fig. 5). If at step 307 the BREAK FLAG is not set, this means that the EXTERN.AL MASTER has attempted to call the local extension unit on his own initiative without having been requested to do so by the local extension unit. In this case, at step 307C a five second timer is set and steps 308 to 309 constitute a loop which checks whether any DTMF digit was received. At the end of the five second time period, if no DTMF digit has been received, then at step 310 DTMF is killed and control reverts to step 302. If, at step 308, a DTMF digit is received, then step 311 checks whether the received DTMF digit corresponds to the code of the current adapter. If not, then step 312 checks whether the received DTMF code is "0". If not, then at step 313 DTMF is killed and control reverts to step 302. However, if DTMF code "0" is received, then step 314 checks whether an additional DTMF digit is received and, if so, the received DTMF digit is stored at step 315, DTMF is killed and control branches to step 316 which initiates the CONNECT ROUTINE described in detail below with reference to Fig. 9 of the drawings. If at step 314 no additional DTMF digit is received, then step 317 allows for step 314 to be repeated if the five second time period has not elapsed or, alternatively, step 318 causes control to branch to step 310, which kills DTMF and reverts control to step 302. If, at step 311, the received code corresponds to the address code of the current handset then, again, the CONNECT ROUTINE is initiated so as to enable connection of the addressed telephone extension unit to the telephone line, thereby changing its status from an EXTERNAL SLAVE to an EXTERNAL MASTER. The significance of step 312, which checks whether the received DTMF code is "0", and associated steps 314 and 315, which allow for a subsequent DTMF digit to be stored, will become clear when the CONNECT ROUTINE is described with reference to Fig. 9 of the drawings. However, briefly at this stage, it allows for the EXTERN.AL MASTER to call a desired EXTERNAL SLAVE by sending a specific number of beeps corresponding to the address code of the desired EXTERNAL SLAVE to all EXTERNAL SLAVES, any one of which can then be lifted by an internal user who recognizes the code as his extension number. In contrast to this, dialling only the DTMF digit of the desired adapter will cause only that adapter to ring and if the user is in a different room he will receive no indication of the attempted transfer. Fig. 7 shows the logic associated with an INTERCOM MASTER routine 400 which is invoked when an extension unit depresses the push¬ button switch 65 prior to lifting the handset, thereby informing the system that an internal call is to be dialled. At step 401, various initialization steps are effected and, in particular, DTMF is awakened and a pulse followed by e.g. "#" is sent down the telephone line, whereafter DTMF is killed. Relay A is set so as to change the coupling of the extension unit to the telephone line from direct coupling to a.c. coupling via the transformer 52 and the capacitor 54a. At step 402, the General Off Hook signal is tested. If ON, this means that another extension unit has already lifted its handset and so at step 403 the timer is reset and the EXTERNAL SLAVE routine 300 is invoked as described above with reference to Fig. 6 of the drawings. Otherwise, only the handset of the current extension unit has been lifted and at step 404 the output of the Ring Detector 46 is evaluated. If ON, an external call is on the line and at step 405 the timer is reset and control branches to the MAIN routine 100, shown in Fig. 4. If no RING signal is detected, then at step 406 the status of the Local Off Hook Detector 50 is determined. If ON, then the local handset has been lifted and at step 407 relay B is set, DTMF is awakened and a DTMF code e.g. "C" is sent for incrementing a USER'S COUNT variable stored in all of the adapters as will be described below. At the same time, a five second timer is set. Otherwise, if at step 406 no Local Off Hook signal is detected, this means that although the push-button switch 65 was depressed, the hand set was not lifted and step 408 allows the loop to be repeated until the five second time period has elapsed whereupon, at step 409, if the handset has still not been lifted control branches to the MAIN routine 100 shown in Fig. 4. However, if the handset were lifted within the five second time period, then an internal extension code may now be dialled in order to call another internal extension. Thus, step 410 checks whether the current telephone extension unit employs pulse dialling and, if not, at step 411 the presence of a DTMF signal is checked. If no DTMF signal is detected then, at step 412, the status of the Local Off Hook Detector is again checked and, if affirmative, step 413 checks for a Ring signal to allow for the possible arrival of an external call. If no Ring signal is detected, then step 414 checks whether the five second time period has terminated, in which case control branches to step 434, which will be described in greater detail below. Otherwise, the loop is repeated either until the five second time period elapses or until the loop exits owing to one of the following conditions. It should be noted that the USER'S COUNTER is incremented in respect of every adaptor in the system 10 whenever a local handset is lifted and the system is in INTERCOM MODE. Consequently, before an external call may be received, it is necessary to reset the USER'S COUNTER in all of the adapters in the system. Thus, if at step 413 a Ring signal is detected, then step 416 checks whether the push-button switch 65 is depressed for the purpose of accepting the external call. If the push-button switch 65 is depressed, then at step 417 one beep is sent, relay B is reset, the INTERCOM FLAG (set at step 401) is reset, relay A is reset and control branches to the MAIN routine 100 shown in Fig. 4 whereby the INTERCOM MASTER is d.c. connected to the telephone line 11 thereby becoming EXTERNAL MASTER so as to allow reception of the external call. Since the EXTERNAL MASTER is off-hook, the General Off Hook Detector 72 produces a General Off Hook signal, responsive to which the USER'S COUNTER is reset in all of the telephone extension units. If, at step 416, the push-button switch 65 is not depressed, then control branches to 410 where the loop is repeated either until the push- button switch 65 in the local extension unit is depressed or, alternatively, until owing to the termination of the five second time period, control branches to step 434. If, at step 412, no Local Off Hook signal is detected, this means that the local handset has been replaced. Therefore, at step 418 DTMF is awakened, a disconnect code "D" is sent, DTMF is killed, the USER'S COUNT is decremented in all of the adapters and relay B is reset. If, at step 419, the USER'S COUNT is greater than "0" then, at step 420, the current control unit branches to step 525 of the INTERCOM SLAVE routine which is described below in greater detail with particular reference to Fig. 8 of the drawings. Otherwise, if the USER'S COUNT is "0", this means that all the local handsets have been replaced, including the current handset, and therefore at step 421, relay A is reset, the INTERCOM FLAG set at step 401 is reset and control branches to the MAIN routine 100 shown in Fig. 4. At step 410, if the test for pulse dial is affirmative, then at step 422 the pulses are converted to DTMF signals, a fifteen second timer is set and beeps are sent along the line. At step 423 the fifteen second timer is set and the beeps are sent if the current handset operates under DTMF. Step 424 checks whether the DTMF code is "C" (i.e. Connect). If not, step 425 checks whether the Local Off Hook signal has been detected. If so, then step 426 checks whether the Ring Detector 46 has detected a Ring signal and, if not, step 427 checks whether the push-button switch 65 has been depressed. If not, step 428 checks whether the fifteen second time period has terminated, the loop starting from step 424 being repeated until the fifteen second time period terminates. If, at step 427, the push-button switch 65 is depressed, then step 429 resets the fifteen second timer, sends a pulse followed by e.g. "#", whereupon control branches to step 410 permitting another internal extension to be dialled by the INTERCOM MASTER. If, at step 428, the fifteen second time period has elapsed, then step 430 kills DTMF, stops the beeps and returns control to step 434. If at step 424, a Connect code (e.g. DTMF "C") is detected, then step 431 resets the timer, stops the beeps and increments the USER'S COUNT in respect of all adapters in the system, whereupon control branches to step 434. If in step 426, a Ring signal is detected, then step 432 examines the status of the push-button switch 65. If the push-button switch 65 is depressed, then step 433 branches to step 417 so as to allow the external caller to make connection. If at step 434 a pulse is detected, this indicates that another internal extension unit (constituting an INTERCOM SLAVE) wishes to join the conversation and has depressed the push-button switch on its handset for alerting INTERCOM MASTER of its desire to make connection. If no pulse is detected, then step 435 checks whether the Local Off Hook signal is detected and, if so, step 436 checks whether a Ring signal has been detected. If not, the possibility exists that another internal extension unit wishes to make connection and has alerted the INTERCOM MASTER of this by depressing its push-button switch, thereby setting a BREAK FLAG at step 445B. Step 436A checks whether the BREAK FLAG is set, in which case the INTERCOM MASTER has ten seconds to permit the connection, failing which the INTERCOM SLAVE will, in any case, effect the required connection. Step 436B checks whether this ten second time period has elapsed, in which case, step 437C resets the BRE.AK FLAG, stops the beeps and returns control to step 434. Otherwise, if the ten second time period has not yet terminated, the status of the push-button switch 65 in the INTERCOM MASTER is checked. If it is not depressed, then control returns to step 434. Otherwise, at step 438 a single beep is generated, the beeps are then stopped, a five second timer is set, DTMF is awakened, a pulse followed by e.g. "#" is sent and control reverts to step 410, permitting the INTERCOM MASTER to dial the extension code for effecting the required connection with the respective INTERCOM SLAVE. If at step 434 a pulse is detected, this could either be due to the fact that another INTERCOM MASTER participating in the internal conversa¬ tion wishes now to disconnect or, alternatively, the reverse: an INTERCOM SLAVE wishes to connect. In either case, step 442 awakens DTMF and sets a one second timer for receiving a DTMF code. Steps 443, 444, 445 and 445A check for the arrival within the one second time period of DTMF codes e.g. "C", "D" and "A" for effecting Connection, Disconnection and Alert, respectively, of an INTERNAL SLAVE handset. If a Connect code is received, then step 446 increases the USER'S COUNT and passes control to step 434, the loop being repeated until the local handset is replaced and control branches to step 418. If a Disconnect code is received, then the USER'S COUNT is decreased and control branches to step 419. If an Alert code is received then, at step 445B, a ten second timer is set, the BREAK FLAG is set and beeps are sent down the telephone line so as to alert the INTERCOM MASTER that a currently disconnected extension unit (INTERCOM SLAVE) wishes to make connection. Control then passes to step 434. Fig. 8 describes the INTERCOM SLAVE routine 500 which parallels the INTERCOM MASTER routine 400 described above with reference to Fig. 7 of the drawings. Therefore, not all steps are described in detail even though, for the sake of completeness, all relevant steps are shown in the figure. At the outset, it will be recalled that the INTERCOM SLAVE routine 500 is executed in one of two conditions: either the INTERCOM MASTER wishes to call a disconnected handset (INTERCOM SLAVE) for participation in an existing conversation; or, alternatively, the INTERCOM SLAVE on its own initiative wishes to join a conversation by sending an Alert signal. In either case, at step 501 a one second timer is set an DTMF is awakened, whereafter step 502 checks for the presence of a DTMF "#" code. If absent, the loop is repeated until step 503 determines the one second time period has elapsed whereafter, at step 504, DTMF is killed and control branches to the MAIN routine 100 (Fig. 4). However, if at step 502 a DTMF "#" code is detected, then step 504 sets a five second timer and awakens DTMF. Step 506 checks for the presence of a General Off Hook signal whose presence, of course, indicates that one of the extension units is connected to the external line. In such conditions, it is not admissible for other extension units to intercommunicate via the INTERCOM MODE and so, at step 507, the USER'S COUNT is reset, relay B is reset and control branches to the EXTERNAL SLAVE routine 300 described above with reference to Fig. 6 of the drawings. If at step 506 no General Off Hook signal is detected, then step 508 checks for receipt of a DTMF "C" code sent by the INTERCOM MASTER for connecting the INTERCOM SLAVE handset. If no DTMF "C" code is detected, then step 509 causes the loop to be repeated until the five second time period for effecting the desired connection terminates, whereupon at step 510 DTMF is killed and control branches to the MAIN routine 100 (Fig. 4). Otherwise, if a DTMF "C" code is detected, then step 511 sets relay A so as to connect the extension unit to the line via the transformer 52 and the capacitor 55, a six second timer is set, DTMF is awakened and the USER'S COUNT is incremented. By this means, the fact that another extension unit has gone off hook, thereby increasing by one the number of off-hook handsets, is recorded in the adaptor of the current INTERCOM SLAVE and ensures that all adapters are, at all times, updated as to the number of handsets which are off-hook in the INTERCOM MODE. This is important because, in INTERCOM MODE, there is no General Off Hook signal and therefore the only way to determine whether any of the handsets is off-hook is to keep a constant tally in all of the adapters of the number of handsets which are off-hook. Only when this number equals zero is it known that all handsets have been returned, thereby allowing control to branch to the MAIN routine 100 (Fig. 4). Steps 512 to 520 allow the INTERCOM SLAVE to monitor an extension code dialled by an INTERCOM MASTER and to effect the required connection if the dialled extension code corresponds to the extension code of the INTERCOM SLAVE. If the dialled extension number is preceded by "0", then a number of beeps corresponding to the dialled extension will be heard at all INTERCOM SLAVES in the system. For example, if the EXTERNAL MASTER wishes to dial extension 5, then either DTMF "5" can be dialled directly, in which case only the addressed extension unit will ring; or, alternatively, DTMF "05" can be dialled, in which case all on-hook adapters in the system will emit five beeps for so long as the timer is set, or until one of the INTERNAL SLAVE handsets is lifted. As has been explained above with reference to the EXTERNAL SLAVE routine, this permits a user to identify that he is being called even when he is not in his room and, further, allows him to join the conversation by lifting any of the on- hook handsets: not necessarily his own. Steps 521 to 524 check whether a DTMF "D" code or a DTMF "C" code is received, indicating that an INTERNAL MASTER has disconnected or that another INTERNAL SLAVE has connected, respectively, the USER'S COUNT being adjusted accordingly. Control branches to step 525 in the event that no DTMF code was received and the five second time period has elapsed. In this case, step 525 kills DTMF and step 526 checks whether a General Off Hook signal is detected. If so, then step 527 resets the USER'S COUNT, resets relay B and branches to the EXTERNAL SLAVE routine 300 (Fig. 6). If, at step 526, no General Off Hook signal is detected, then step 528 checks for the presence on the line of a 0V pulse followed by "#" which, it will be recalled, are produced by an INTERCOM MAPSTER depressing its push-button switch 65 prior to dialling ^he extension code of a INTERNAL SLAVE to which connection is required. If, at step 529, the BREAK FLAG is not set, this means that the INTERNAL MASTER initiated the connection, in which case a five second timer is set, DTMF is awakened and control reverts to step 512, allowing the INTERCOM MASTER to dial the required extension code. Otherwise, if at step 529 the BREAK FLAG is set, this means that the current adapter initiated the connection by sending an .Alert signal and that the INTERCOM MASTER responded within the ten second time period so as to allow the INTERCOM SLAVE to effect the required connection. In this case, control branches to step 534, wherein the DTMF beeps are stopped and relay B is set, thereby connecting the current telephone extension unit which thus ceases to be an INTERCOM SLAVE and becomes instead an INTERCOM MASTER, control branching to step 434 in the INTERCOM MASTER routine (see Fig. 7). Fig. 9 shows the CONNECT routine 600 which is called by the EXTERNAL SLAVE and INTERCOM SLAVE routines described above with reference to Figs. 6 and 8 of the drawings. At step 601 a fifteen second timer is set for allowing connection of the current extension unit within a fifteen second time period. At step 602, DTMF is killed whilst step 602 distinguishes between the current adapter being addressed directly by dialling its extension code only or, alternatively, to its being addressed by first dialling "0". If at step 603 no "smart code" is detected, then step 603A checks for the presence on the line of DTMF "D" denoting disconnection of an INTERNAL MASTER extension unit. If DTMF "D" is detected, then step 603B checks whether the USER'S COUNT is zero, in which case the beeps are stopped and control branches to the MAIN routine 100 (see Fig. 4). Otherwise, step 603D stops the beeps and reverts control to the INTERCOM SLAVE routine 500 shown in Fig. 8. If, at step 603A, no DTMF "D" code is detected, then step 604 checks for the present of the Local Off Hook signal and, if this is absent, step 604A checks the status of the INTERCOM FLAG. If this is not set, then the system 10 is not in the INTERCOM MODE and so step 604B checks whether a General Off Hook signal is detected. If not, step 604C stops the beeps and restores control to the MAIN routine 100 (Fig. 4). Otherwise, if a General Off Hook signal is detected, this means that the CONNECT routine has been called by the EXTERNAL SLAVE routine 300 (Fig. 6). Regardless of which routine calls the CONNECT routine 600, the loop is repeated, as required, until the fifteen second time period terminates or until step 603 detects a "smart code", or until step 604 detects a Local Off Hook signal. If at step 605 the fifteen second time period has terminated, then step 606 stops the beeps and steps 607, 608 and 609 cause branching to step 525 of the INTERNAL SLAVE routine or to step 300 of the EXTERNAL SLAVE routine, depending on which of these two routines called the CONNECT routine in the first place. If at step 603 a "smart code" is detected, then steps 610, 611 and 612 revert control to step 501 of the INTERCOM SLAVE routine or to step 305 of the EXTERNAL SLAVE routine, again depending on which of these two routines called the CONNECT routine. Otherwise, if at step 604 a Local Off Hook signal is detected, then steps 613, 614 and 615 stop the beeps and enable the push-button switch. Further, step 614 sets relay B, increases the USER'S COUNT, and sends DTMF "C" for connecting the INTERCOM SLAVE, whereupon control branches to step 434 of the INTERCOM MASTER routine shown in Fig. 7. Otherwise, for an EXTERNAL SLAVE, relay A is reset and control branches to the EXTERNAL MASTER routine. In the system described so far, the default condition is EXTERNAL MASTER such that any extension unit can initiate an external call by lifting the handset so as to couple the extension unit directly to the outside line 11, whereupon an external extension number can be dialled. In this configura¬ tion, depressing the push-button switch prior to lifting the handset puts the adaptor into INTERCOM MODE. However, it will readily be appreciated that this is merely a design choice and it is equally feasible for the default condition to be the INTERCOM MODE in which the default condition for an extension unit is INTERCOM MASTER such that EXTERNAL MASTER must be selected by lifting the handset and dialling "9", for example. Fig. 10 shows the principal steps which are invoked in such an arrangement in a so-called MAIN NORMALLY DISCONNECTED routine 700. Step 701 initializes the adaptor, disconnects relay A and connects relay B, and sets the INTERCOM FLAG. Thereafter, step 703 checks whether a Local Off Hook signal is detected and, if not, step 704 checks whether a 0V pulse is detected on the line. If not, step 705 checks whether the Ring Detector has detected a ring signal and, if not, step 706 checks whether a General Off Hook signal has been detected. If at step 704 a 0V pulse is detected, this means that an INTERCOM MASTER wishes to call one of the on-hook extension units and control therefore branches, at step 707, to the INTERNAL SLAVE routine at step 501. Otherwise, if at step 705 a ring signal is detected, then at step 708 relay B is disconnected and relay A is connected, and control branches to the EXTERNAL MASTER routine. If at step 706 a General Off Hook signal is detected, this means that another handset has initiated an external call and in order to ensure privacy, step 709 disconnects relay B and branches to step 302 of the EXTERNAL SLAVE routine shown in Fig. 6. If at step 703 a Local Off Hook signal is detected, then control branches to step 710 wherein the status of the push-button switch is determined. If this is not depressed, then step 711 checks whether "9" was dialled. This caters for the situation wherein all the local extension units are normally connected in INTERCOM MODE and an outside line is obtained by first dialling "9". If no "9" is detected, then step 712 checks whether a Local Off Hook signal is detected so as to check whether the handset of the current extension unit has been replaced, in which case control reverts to step 703. If at step 710 a push-button signal is detected, then step 716 sends a "smart code" followed by DTMF "C" and control branches to the INTERCOM MASTER routine shown in Fig. 7. Otherwise, if at step 711 the "9" is detected, this means that an outside line has been dialled and step 717 disconnects relay B, connects relay A and branches to step 202 of the EXTERNAL MASTER routine. If at step 713 a zero volt pulse is detected, then at step 718 control branches to step 501 of the INTERCOM SLAVE routine (Fig. 8). Otherwise, if at step 714 no Ring signal is detected, then step 715 checks whether a General Off Hook signal is detected. If not, then control reverts to step 703. The presence of a Ring signal at step 714 indicates that an external call is on the line, in which case step 719 disconnects relay B, connects relay A and reverts control to the EXTERNAL .MASTER routine (Fig. 5). If at step 715 a General Off Hook signal is detected, this means that an external call is already in progress, in which case step 720 disconnects relay B and reverts control to step 302 of the EXTERNAL SLAVE routine, whereupon the current extension unit ceases to be an INTERCOM MASTER and becomes instead an EXTERNAL SLAVE. Although the invention has been described with particular reference to a telephone system providing access to both external and internal extensions, it may also be used for internal-only systems such as field telephones and the like. For example, field telephones provide a single internal line connected to a plurality of extensions which do not have extension numbers. To "dial" an extension, a handle is usually turned or a button depressed, and this generates a call signal which is heard by all the interconnected extensions. Such a system offers no privacy because all extensions can participate in the conversation merely by lifting their respective handsets. By connecting the extensions to the internal line via the adapter according to the invention, a field telephone of this kind can be converted into an intercom system wherein each extension has a unique extension number and privacy is ensured during a conversation between two or more users such that other users (i.e. "slaves") cannot hear the conversation by lifting their handsets. Since the system is internal only, the coupling provided by the adapters may be a.c. or d.c. It should be understood, however, that the adapters which are required for such a system are merely a simplified form of the adapter described in detail above with particular reference to Fig. 2 of the drawings. Specifically, in the simplified adapter for use with a field telephone system, the relay B, the transformer 52, the Local Off Hook Detector 50 and the General Off Hook Detector 72 can all be dispensed with, and the micro-controller logic altered accordingly. The preferred embodiment allows for addressable extension units which each have a unique extension number. However, it will be under¬ stood that this is not mandatory and can be dispensed with in such situations as the conventional field telephone just described or where, for example, only two extensions are connected to the line. Furthermore, although in the preferred embodiment the a.c. coupling circuit includes a transformer and a capacitor, it will be understood that other configurations, such as a single capacitor, are also suitable. It will also be understood that the DTMF codes e.g. "#", "A", "C" and "D" which are employed for call transfer as well as alerting, connecting and disconnecting of extension units are by way of example only. Other DTMF codes may equally well be employed consistent with the detailed logic described above. It will further be appreciated that, whilst relays A and B are shown in the preferred embodiment for operating the various switches, solid state components such as triacs can be used instead. In order to comply with telephone company regulations, such triacs should preferably be disabled when energized. It will equally well be appreciated that although the remotely- operated switch 67 is shown as normally open, it can also be normally closed. Likewise, multi-pole, multi-throw switches can be controlled remotely and the switch can be electro-mechanical or solid state or, indeed, any type of switch. It should also be noted that the invention includes within its scope a modified telephonic device including therein all the relevant components described above with respect to the adapter. Obviously, in such case the adapter can be dispensed with. Furthermore, combining the adapter within the telephonic device itself avoids unnecessary duplication of those components, such as an extension off hook detector, an audio signalling device and so on, which are required within the adapter but are provided in any case within telephonic device. In the context of the specification and appended claims, the term "telephonic device" includes within its scope any device connected to a telephone line such as, for example, a telephone extension unit, a facsimile machine, an auto answering machine and so on. The invention thus provides for a comprehensive telephone exchange system to be constructed merely by plugging an adaptor between each telephone extension unit and a corresponding telephone outlet socket, there being no requirement to install an interface unit at the point where the telephone line enters the system. .Although the logic has been described in detail for connecting telephone extension units to the system in either an EXTERNAL or INTERCOM mode, it will readily be appreciated that the telephone extension unit can be replaced, as required, by a facsimile machine, a modem, an answering machine, a remotely-operated appliance, and so on. In such case, a standard telephone extension unit can be addressed by dialling the appropriate extension code or any of these appliances can be accessed in response to appropriate remote handshake protocols or, in the case of an answering machine, in the absence of a handset being lifted within a predetermined number of rings.

Claims: CLAIMS: 1. A telephone system, comprising: at least two telephone outlet sockets coupled to a telephone line across which there is applied a d.c. voltage, a respective telephone extension unit for coupling to the telephone line, and a respective switching means connected between each one of the telephone outlet sockets and the corresponding telephone extension unit coupled thereto and switchable between a first position in which the telephone extension unit is directly connected to the telephone outlet socket and a second position wherein the telephone extension unit is a.c. coupled to the telephone outlet socket so as to block said d.c. voltage from the telephone extension unit; the system further including in respect of each telephone extension unit: call signalling means for sending a call signal to another telephone extension unit, a call signal detector for detecting said call signal sent by a remote telephone extension unit and producing a call detection signal, a call indication means responsive to the call detection signal for generating a call indication signal, a status indication means responsive to a status of the system for producing a respective status indication signal, an actuation means for producing an actuation signal, and a control unit coupled to the status indication means, the actuation means, the call signal detector, the call indication means, the call signalling means and the switching means and being responsive to the status indication, the actuation and the call detection signals, for operating the call indication means, the call signalling means and the switching means. 2. The system according to Claim 1, wherein the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line. 3. The system according to Claim 1, wherein: the status indication means includes a general off hook detector for detecting a general off-hook condition in any one of the telephone extension units and producing a general off hook signal. 4. The system according to Claim 3, wherein: the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line, the status indication means further includes an extension off hook detector for detecting an extension off hook condition in the respective telephone extension unit and producing an extension off hook signal, and each of the control units is responsive to said general off hook signal and to an absence of said extension off hook signal for switching the respective switching means to the third position. 5. The system according to Claim 4, wherein: each of the telephone extension units has a unique extension number, the call signalling means includes encoding means for sending a code down the telephone line in respect of the unique extension number corresponding to another telephone extension unit, and each of the control units is further responsive to the actuation signal following the extension off hook signal for enabling the call signalling means. 6. The system according to Claim 4, wherein each of the control units is responsive to the actuation signal preceding the extension off hook signal for switching the respective switching means to the second position and sending a predetermined code down the telephone line. 7. The system according to Claim 4, wherein each of the control units is responsive to the actuation signal preceding the extension off hook signal for switching the respective switching means to the first position and sending a predetermined code down the telephone line. 8. The system according to Claim 4, wherein: the call indication means includes a ring tone generator for generating a pilot tone indicative of the presence of an external incoming call, and during an internal connection between at least two telephone extension units each of the respective control units in said at least two telephone extension units is further responsive to an actuation signal generated by any one of the respective actuation means therein for switching the respective switching means to the first position so as to d.c. connect the respective telephone extension unit to the respective telephone outlet socket. 9. The system according to Claim 4, wherein during an external connection between a first internal caller at a first telephone extension unit and an external caller, the control unit coupled to the first telephone extension unit is responsive to an actuation signal generated by the actuation means coupled to a second telephone extension unit for producing an alert signal for indicating to the first internal caller that a second internal caller wants to use the system. 10. The system according to Claim 9, wherein the control unit coupled to the second telephone extension unit is responsive to an actuation signal generated by the actuation means coupled to the first telephone extension unit for switching the switching means coupled to the second telephone extension unit to the first position. 11. The system according to Claim 9, wherein: the control unit coupled to the second telephone extension unit includes a timing means for producing an elapsed time signal a predetermined time interval after said actuation signal is generated, and the control unit is responsive to the elapsed time signal for switching the respective switching means to the first position. 12. The system according to Claim 4, comprising at least three telephone extension units, wherein during an internal connection between respective first and second internal callers at the first and second telephone extension units, the respective control units coupled to the first and second telephone extension units are each responsive to an actuation signal generated by the actuation means coupled to a third telephone extension unit for producing a respective warning signal for indicating to the first and second internal callers that a third internal caller wants to use the system. 13. The system according to Claim 12, wherein the control unit coupled to the third telephone extension unit is responsive to an actuation signal generated by either of the respective actuation means coupled to the first and second telephone extension units for switching the switching means coupled to the third telephone extension unit to the second position. 14. The system according to Claim 12, wherein the control unit coupled to the third telephone extension unit includes a timing means for producing an elapsed time signal a predetermined time interval after said actuation signal is generated, the control unit being responsive to the elapsed time signal for switching the respective switching means to the second position. 15. The system according to Claim 3, wherein: the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line, the control unit is responsive to the respective general off hook signal and the absence of the extension off hook signal for switching the switching means to the third position. 16. The system according to Claim 6, wherein: the control unit is responsive to said predetermined code for switching the switching means to the third position. 17. The system according to Claim 16, wherein said predetermined code is a zero volt pulse followed by a dialled number. 18. The system according to Claim 5, wherein: the call indication means is an audible device, and the call signal is coded so as to produce in each disconnected telephone extension unit an audible signal characteristic of a desired telephone extension unit. 19. The system according to Claim 1, wherein each control unit is further coupled to a respective loudspeaker responsive to the call detection signal for relaying a telephone call audibly. 20. The system according to Claim 1, wherein: at least one of the telephone extension units is a remotely-operated appliance responsive to an enabling signal for operating, and the control unit is responsive to the call detection signal for generating the enabling signal. 21. The system according to Claim 20, wherein the appliance is a remotely-operated switch. 22. The system according to Claim 1, further including a controller for allowing connection and control of all automatic answering devices connected to the telephone line, the controller comprising: a switching means connected between the controller and the telephone line and switchable between a first position in which the controller is directly connected to the telephone line and a second position wherein the controller is a.c. coupled to the telephone line so as to block said d.c. voltage from the controller, a call signal detector operating subsequent to a general off hook condition for detecting a device-type signal characteristic of a desired automatic answering device which is to be coupled to the line and generat¬ ing a respective address signal, a status indication means responsive to a status of the system for producing a respective status indication signal, call signalling means responsive to the address signal for sending a corresponding call signal to said desired automatic answering device, and a control unit coupled to the status indication means, the call signal detector, the call signalling means and the switching means and being responsive to the status indication signal, and the address signals, for operating the call signalling means and the switching means. 23. The system according to Claim 22, wherein: the status indication means includes a general off hook detector for detecting a general off hook condition in any one of the telephone extension units and producing a general off hook signal, there is further included a ring detector responsive to an external ring signal for generating a corresponding ring detection signal, and the control unit is coupled to the ring detector and is responsive to the ring detection signal for counting a cumulative number of ring signals so as to generate a General Off Hook condition after a predetermined number of ring signals. 24. The system according to Claim 1, wherein the call signal detector is normally dormant and is responsive to an awakening signal for operating. 25. The system according to Claim 24, wherein: the actuation signal includes a zero volt pulse, and the call signal detector is coupled to a zero volt pulse detector for detecting the zero volt pulse and producing said awakening signal. 26. The system according to Claim 1, wherein in said second position of the switching means the respective telephone extension unit is connected to the telephone outlet socket via a capacitive circuit. 27. The system according to Claim 26, wherein said capacitive circuit further includes a transformer. 28. A telephone adapter, comprising: line coupling means for coupling to a telephone line having a d.c. voltage across a pair of terminals thereof, an extension coupling means for coupling to a telephone extension unit, and a switching means connected between the line coupling means and the extension coupling means and operable between a first position in which the telephone extension unit is directly connected to the telephone outlet socket and a second position in which the extension coupling means is a.c. coupled to the line coupling means so as to block said d.c. voltage when the adapter is connected to the telephone outlet socket. 29. The telephone adapter according to Claim 28, wherein the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line. 30. The telephone adapter according to Claim 28, further including: call signalling means for sending a call signal to another telephone adapter, a call signal detector for detecting said call signal sent by a remote telephone adapter and producing a call detection signal, a call indication means responsive to the call detection signal for generating a call indication signal, a status indication means responsive to a status of all adapters connected to the telephone line and for producing a respective status indication signal, an actuation means for producing an actuation signal, and a control unit coupled to the status indication means, the actuation means, the call signal detector, the call indication means, the call signalling means and the switching means and being responsive to the status indication, the actuation and the call detection signals, for operating the call indication means, the call signalling means and the switching means. 31. The telephone adapter according to Claim 30, wherein: the status indication means includes a general off hook detector for detecting a general off hook condition in any telephone adapter connected to the telephone line and producing a general off hook signal. 32. The telephone adapter according to Claim 30, wherein: the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line, the status indication means further includes an extension off hook detector for detecting an extension off hook condition in the respective telephone extension unit and producing an extension off hook signal, and the control unit is responsive to said general off hook signal and to an absence of said extension off hook signal for switching the switching means to the third position. 33. The telephone adapter according to Claim 32, wherein: the telephone adapter has a unique extension number, the call signalling means includes encoding means for sending a code down the telephone line in respect of the unique extension number corresponding to another adapter, and the control unit is further responsive to the actuation signal following the extension off hook signal for enabling the call signalling means. 34. The telephone adapter according to Claim 32, wherein the control unit is responsive to the actuation signal preceding the extension off hook signal for switching the respective switching means to the second position and sending a predetermined code down the telephone line. 35. The telephone adapter according to Claim 32, wherein the control unit is responsive to the actuation signal preceding the extension off hook signal for switching the respective switching means to the first position and sending a predetermined code down the telephone line. 36. The telephone adapter according to Claim 32, wherein: the call indication means includes a ring tone generator for generating a pilot tone indicative of the presence of an external incoming call, and during an internal connection between at least two telephone extension units each coupled to the telephone line via respective adapters each of the respective control units is responsive to an actuation signal generated by any one of the respective actuation means for switching the respective switching means to the first position so as to d.c. connect said at least two telephone extension units to the telephone line. 37. The telephone adapter according to Claim 32, wherein during an external connection between an external caller and a first internal caller at a first telephone extension unit having a first adapter, the control unit coupled to the first adapter is responsive to an actuation signal generated by an actuation means coupled to a second adapter for producing a request signal for indicating to the first internal caller that a second internal caller wants to use the telephone line. 38. The telephone adapter according to Claim 37, wherein the control unit coupled to the second adapter is responsive to an actuation signal generated by the actuation means coupled to the first adapter for switching the switching means coupled to the second adapter to the first position. 39. The telephone adapter according to Claim 37, wherein: the control unit coupled to the second adapter includes a timing means for producing an elapsed time signal a predetermined time interval after said actuation signal is generated, and the control unit is responsive to the elapsed time signal for switching the switching means to the first position. 40. The telephone adapter according to Claim 33 for coupling a first telephone extension unit to a telephone line having at least second and third telephones extension units coupled thereto via respective first, second and third adapters, wherein: during an internal connection between respective first and second internal callers at the first and second adapters, the respective control units coupled to the first and second adapters are each responsive to an actuation signal generated by an actuation means coupled to the third telephone adapter for producing a respective warning signal for indicating to the first and second internal callers that a third internal caller wants to use the telephone line. 41. The telephone adapter according to Claim 40, wherein the control unit coupled to the third telephone adapter is responsive to an actuation signal generated by either of the respective actuation means coupled to the first and second telephone adapters for switching the switching means coupled to the third telephone adapter to the second position. 42. The telephone adapter according to Claim 40, wherein: the control unit coupled to the third telephone adapter is connected to a timing means for producing an elapsed time signal a predetermined time interval after said actuation signal is generated, and the control unit is responsive to the elapsed time signal for switching the switching means to the second position. 43. The telephone adapter according to Claim 31, wherein: the switching means is switchable to a third position wherein the respective telephone extension unit is disconnected from the telephone line, and the control unit is responsive to the general off hook signal and to an absence of the extension off hook signal for switching the switching means to the third position. 44. The telephone adapter according to Claim 34, wherein: the control unit is responsive to said predetermined code for switching the switching means to the third position. 45. The telephone adapter according to Claim 44, wherein said predetermined code is a zero volt pulse followed by a dialled number. 46. The telephone adapter according to Claim 33, wherein: the call indication means is an audible device, the call signal is coded so as to produce in each disconnected adapter an audible signal characteristic of a desired telephone adapter. 47. The telephone adapter according to Claim 30, wherein the control unit is further coupled to a loudspeaker responsive to the status indication signal for relaying a telephone call audibly. 48. The telephone adapter according to Claim 30, wherein there is coupled to thereto at least one remotely-operated appliance responsive to an enabling signal for operating, and the control unit is responsive to the call detection signal for generating the enabling signal. 49. The telephone adapter according to Claim 48, wherein the appliance is a remotely-operated switch. 50. The telephone adapter according to Claim 28, being a controller for allowing connection and control of all automatic answering devices connected to the telephone line, the controller comprising: a switching means connected between the controller and the telephone line and switchable between a first position in which the controller is directly connected to the telephone line and a second position wherein the controller is a.c. coupled to the telephone line so as to block said d.c. voltage from the controller, a call signal detector operating subsequent to a general off hook condition for detecting a device-type signal characteristic of a desired automatic answering device which is to be coupled to the line and generat¬ ing a respective address signal, a status indication means responsive to a status of the system for producing a respective status indication signal, call signalling means responsive to the address signal for sending a corresponding call signal to said desired automatic answering device, and a control unit coupled to the status indication means, the call signal detector, the call signalling means and the switching means and being responsive to the status indication signal, and the address signals, for operating the call signalling means and the switching means. 51. The telephone adapter according to Claim 50, wherein: the status indication means includes a general off hook detector for detecting a general off hook condition in any adapter connected to the telephone line and producing a general off hook signal, there is further included a ring detector responsive to an external ring signal for generating a corresponding ring detection signal, and the control unit is coupled to the ring detector and is responsive to the ring detection signal for counting a cumulative number of ring signals so as to generate a general off hook condition after a predetermined number of ring signals. 52. The telephone adapter according to Claim 30, wherein the call signal detector is normally dormant and is responsive to an awakening signal for operating. 53. The telephone adapter according to Claim 52, wherein: the actuation signal includes a zero volt pulse, and the call signal detector is coupled to a zero volt pulse detector for detecting the zero volt pulse and producing said awakening signal. 54. The telephone adapter according to Claim 30, wherein in said second position of the switching means the respective telephone adapter is connected to the telephone outlet socket via a capacitive circuit. 55. The telephone adapter according to Claim 54, wherein said capacitive circuit further includes a transformer. 56. A telephone system, comprising: a telephone line, at least two telephone extension units for coupling to the telephone line, and a respective switching means connected between each one of the telephone extension units and the telephone line and switchable between a first position in which the telephone extension unit is coupled to the telephone line and a second position in which the telephone extension unit is disconnected from the telephone line; the system further including in respect of each telephone extension unit: call signalling means for sending a call signal to another telephone extension unit, a call signal detector for detecting said call signal sent by a remote telephone extension unit and producing a call detection signal, a call indication means coupled to the call signal detector and responsive to the call detection signal produced thereby for generating a call indication signal, a status indication means responsive to a status of the system for producing a respective status indication signal, and a control unit coupled to the status indication means, the call signal detector, the call indication means, the call signalling means and the switching means and being responsive to the status indication, the actuation and the call detection signals, for operating the call indication means, the call signalling means and the switching means. 57. A telephone adapter, comprising: line coupling means for coupling to a telephone line, an extension coupling means for coupling to a telephone extension unit, and a switching means connected between the line coupling means and the extension coupling means and operable between a first position in which the extension coupling means is coupled to the line coupling means and a second position in which the telephone extension unit is disconnected from the telephone line. 58. The adapter according to Claim 30 being integral with, a telephonic device.

Inventors: NEDIVI DROR, WEINER AVISH JACOB, NEDIVI, DROR, WEINER, AVISH, JACOB
Applicants: EISENBERG & CO USA AGENCY, NEDIVI DROR, WEINER AVISH JACOB, EISENBERG & CO. U.S.A. AGENCY INC., NEDIVI, DROR, WEINER, AVISH, JACOB
Application year: 1994
Main classification: H04M3
IPC codes: H04M1, H04M9, H04M11